1. Field of the Invention
The present invention relates to a device for transmitting optical signals between units that are rotatable relative to each other (a rotary coupling). This is for simultaneously transmitting a plurality of optical signals along a plurality of channels.
2. Description of Related Art
Various transmission systems are known for transmitting optical signals between units that are rotatable relative to each other.
An optical rotary joint for a plurality of channels, comprising a Dove prism, is disclosed in U.S. Pat. No. 5,568,578. In order to attain low attenuation during transmission, low reflection at a reflecting face is necessary, apart from a maintenance of mechanical tolerances, and as good as possible a bundling of light rays by the collimators. This optimization of bundling is not discussed in the mentioned document it is usually achieved with total reflection. For this, a Dove Prism of optically transparent material having a high refractive index is surrounded by air having a substantially lower refractive index.
Furthermore, Abbe-Koenig prisms are known for derotating images. With these prisms too, a deflection of a beam is effected by total reflection at reflecting faces.
Derotating elements of prior art are preferably used in air. However, there are fields of application, for example under high ambient pressure, in which a derotating element must be used in a different medium, such as oil for example. Now for this, the attenuation of a derotating element, caused by reflection at a boundary face between the glass of the prism and the surrounding oil, depends upon the nature and quality (purity) of the oil. Because of fluctuations of the optical properties of the oil, the oil must be assumed to have a higher refractive index when the prism is being dimensioned, to be on the safe side. With this, the angle of reflection of the light beams becomes flatter and the prism must be designed to be longer.